1. Field of the Invention
The present invention relates to a production method of a β-type sialon fluorescent substance.
2. Description of the Related Art
Light-emitting devices combined with a light-emitting element emitting a primary light and a fluorescent substance absorbing the primary light and emitting a secondary light have been drawing attentions as a next-generation light-emitting device being expected to have low power consumption, miniaturization, high brightness and extensive color reproducibility, and they have actively been researched and developed.
For example, there is disclosed a white LED obtaining white light by color mixture of light emitted by the semiconductor light-emitting element and wavelength-converted light by the fluorescent substance, by combining a semiconductor light-emitting element emitting a visible light of short wavelength from blue to purple with a fluorescent substance.
As output of a white LED increases, heat stability and durability of a fluorescent substance have been demanded more than ever, and a fluorescent substance which is low in deterioration of luminescence intensity due to temperature rise and excellent in durability has been required, so a fluorescent substance of nitride or acid nitride typified by a β-type sialon fluorescent substance whose crystal structure is stable has been drawing attentions.
It has been known that a β-type sialon fluorescent substance is obtained by mixing silicon nitride (Si3N4), aluminum nitride (AlN) and an optically-active element compound such as europium oxide (Eu2O3) in a predetermined mole ratio, then burning it at a temperature near 2000° C., and grinding the resulting burned product, or produced by further acid treatment of the burned product obtained (Patent document 1).
However, since the β-type sialon fluorescent substance obtained by the above-described method is low in luminescence intensity, in the case of a white LED obtained by combination with a semiconductor light-emitting element, there is pointed out a problem that the light-emitting efficiency is low.
In order to improve the luminescence intensity of a β-type sialon fluorescent substance, there is a proposition that before burning, fluoride, chloride, iodide or bromide of an element selected from Li, Na, K, Mg, Ca, Sr and Ba, or phosphate is added (Patent document 2). This method aimed to improve reactivity in burning and promote the growth of crystal grain by adding the above-described compound.